A group of business lobbyists warn British Prime Minister Theresa May against perusing a “hard Brexit,” calling on her to secure the UK’s access to the European Union market.

On Friday, the business lobbies signed an open letter in which they urged the government to “make sure that the terms of the deal to leave [the EU] ensure stability, prosperity and improved living standards.”

They called for the government to "give certainty to business by immediately ruling this option under any circumstances,” referring to the so-called “hard Brexit,” which suggests surrendering access to the European Union’s single market and scrapping free movement of EU nationals in return for securing control over immigration.

On June 23, some 52 percent (17.4 million) of British people voted to leave the EU after 43 years of membership.

The vote called Brexit sent shockwaves throughout the world and prompted the EU members to trigger Article 50 of the Lisbon Treaty -- the step required to officially begin the withdrawal -- as soon as possible in an effort to end what they called political and economic uncertainty

Starting the negotiations would begin a two-year countdown for the UK to separate itself from Brussels.

The letter also suggested EU negotiations will not be completed within two-year time-frame.

This is a notice board displaying the exchange rate for pound sterling to the Euro and other currencies, outside a money exchange store in central London on June 25, 2016. (Photo by AFP)

One of the signatories, director-general of the Confederation of British Industry, Carolyn Fairbairn called for “ruling out of the really worst options, to reassure investors that the UK was still a really good place to invest".

“It is the investment decisions that are being taken now that make this urgent," Fairbairn added.

Earlier this week, May said she wanted to give the country’s businesses the "maximum freedom" to trade with and operate within the union, when she starts formal exit talks early next year.

Brexit Secretary David Davis, however, reaffirmed that single market membership was "very improbable" if it meant "giving up control of our borders."

Leave campaigners insist that Brexit is a sovereign decision that should be completed as quickly as possible. They say the option of soft Brexit, which was introduced by those who had supported the country’s remaining in the union, would keep Britain's borders open to refugees.